I think the answer is in your question. It is adorable when Gillian shares pictures of the set or bts with the fandom because she is the fandom. We’re fan of her, of the show she plays in, of her relationship with David. She is in control of what she shares with us, she’s advised, she knows what she does, it is her job. Monique is just sharing bits of David’s private life and places to whomever she decides to accept then block, then accept again.
Following Gillian on social media is just following Gillian. But following Monique is stalking. That’s the difference.